1. Field of the Invention
This invention relates generally to optical tuners and more particularly to silicon-based tunable waveguides for an optical bench router.
2. Description of the Related Art
In order for a semiconductor layer to be optically tunable, it must have properties that allow either the index of refraction to change when injected with current, or its optical length to change. Normally Group III-V elements are used as the substrate for an optically tunable device since these elements change their refractive index upon current injection. Lithium Niobate (LiNbO.sub.3) is also used as a tunable substrate because of its optical refractive properties.
In most semiconductor devices, silicon-based elements are the preferred material to use because of its ease of fabrication, relative inexpensiveness, and ease of coupling to fiber optic networks. Pure silicon, as a substrate for an optical waveguide, makes a poor choice as it is not tunable. Its optical length does not change upon current injection nor is its refractive index easily variable. Silicon is virtually inert to current injection and therefore not viable as a tunable substrate. The only way to change silicon's refractive index is by heat. This is not a reliable method, nor is it highly controllable.
Hydrogenated amorphous silicon (a-Si:H) is a silicon based semiconductor material that, as reported in "Enhanced Electro Optic Effect in Amorphous Hydrogenated Silicon Based Waveguides" by M. Zelikson et al., Applied Physics Letters, Vol. 61, No. 14, Oct. 5, 1992, pages 1664-1666, has been shown to have tunable properties. The waveguide becomes tunable when current is injected across the material. The amount of tuning will vary depending upon the length of the waveguide as well as the amount of current. The length of the waveguide will cause the delay of the light to vary.
A Dragone Router is one use of a waveguide that normally is not tunable. This is a multiplexer that is created by using multiple waveguides. By interconnecting two star couplers with a star waveguide grating, it has been demonstrated that a monolithic 15.times.15 multiplexer on silicon can be built. This device can become tunable if instead, the multiplexer is built on Indium Phosphate (InP). See "Demonstration of a 15.times.15 Arrayed Waveguide Multiplexer on InP" by M. Zirngibl et al. IEEE Photonics Tech. Letters, Vol. 4, No. 11, November 1992, pages 1251-53 and U.S. Pat. No. 5,243,672 to C. Dragone. It would be desirable to be able to build a tunable multiplexer using a silicon-based substrate as its bench or active region. This would allow tunability without the high cost of coupling the optics to a Group III-V material or InP. The high cost manifests itself because of the limitations to the waveguide dimensions at the coupling point of the prior art materials. This leads to a certain amount of mismatching between the waveguide and the optics.